The present disclosure relates to an optical recording medium. More specifically, the present disclosure relates to an optical recording medium including an inorganic recording layer.
Hitherto, Compact Discs (CDs), Digital Versatile Discs (DVDs), and the like have led the market of optical recording media. However, recently, with a realization of high-vision televisions and a rapid increase in the volume of data handled by personal computers (PCs), a further increase in the capacity of optical recording media has been desired. In order to meet this desire, large-capacity optical recording media for a blue laser, such as Blu-ray Discs (BDs) (registered trademark), have appeared, and a new market of large-capacity optical recording media has been established.
Recordable optical recording media include rewritable optical recording media typified by a Digital Versatile Disc-Recordable (DVD-R) and a Digital Versatile Disc±ReWritable (DVD±RW), and write-once optical recording media typified by a Compact Disc-Recordable (CD-R) and a Digital Versatile Disc-Recordable (DVD-R). In particular, the latter write-once optical recording media have made a large contribution to the market expansion as low-price media. Accordingly, in order to expand the market also in large-capacity optical recording media for a blue laser, it is necessary to reduce the price of the write-once optical recording media. Furthermore, in general, it is believed that optical recording media have high storage reliability from the standpoint of the principle of recording and reproduction thereof, as compared with hard disk drives (HDDs) and flash memories. For example, optical recording media have started to be used for storing important information, and thus a demand for optical recording media as archival media has been recently increasing.
In write-once optical recording media, both inorganic materials and organic dye materials are permitted as recoding materials. Write-once optical recording media including an organic material are advantageous in that they can be produced by a spin coating method at a low cost. On the other hand, write-once optical recording media including an inorganic material are advantageous in that they are good in terms of reproduction durability and formation of multiple recording layers, but are disadvantageous in that a large sputtering apparatus is necessary. Accordingly, in order that such write-once optical recording media including an inorganic material can be competitive with write-once optical recording media including an organic material in terms of cost, it is necessary to suppress the initial investment for a production apparatus, to improve the production takt per disc, and to efficiently produce the recording media.
An example of the most effective method for solving the above problem is reducing the number of layers constituting a recording film to reduce the number of deposition chambers, thus reducing the initial investment for a sputtering apparatus and reducing the production takt. However, even when the number of layers is simply reduced, if a film having a large thickness is deposited using a material whose deposition rate is low, the production takt increases, which may actually result in an increase in the cost.
Hitherto, transparent dielectric materials such as SiN and ZnS—SiO2 have been mainly used as materials of protective layers of write-once optical recording media including an inorganic material (refer to, for example, Japanese Unexamined Patent Application Publication No. 2003-59106). Although SiN and ZnS—SiO2 are advantageous in that the deposition rates thereof are high and thus these materials are good in terms of productivity, these materials have a problem of a poor storage characteristic (storage reliability) of recorded data. Some dielectric materials other than the above materials have high storage reliability. However, it is necessary to deposit these dielectric materials by radio-frequency (RF) sputtering, and thus the deposition rates thereof are very low. Thus, these dielectric materials have a problem of low productivity. Thus, it is very difficult to satisfy both high storage reliability and productivity.
To solve this problem, for example, Japanese Unexamined Patent Application Publication No. 2009-129526 has proposed a technology of using, as a material of a protective layer, indium oxide and tin oxide (hereinafter referred to as “ITO”), which can be deposited by direct-current (DC) sputtering. In this technology, high storage reliability can be realized in write-once optical recording media, and the number of deposition chamber can be reduced to achieve high productivity. Specifically, since a high deposition rate can be achieved by using ITO as a material of the protective layer, the thickness of the protective layer can be increased even in a single chamber. As a result, the number of chambers can be reduced while maintaining the degree of freedom of the design of the optical recording media.
In the technology described in Japanese Unexamined Patent Application Publication No. 2009-129526, it is possible to realize storage reliability at a sufficient level in daily use. However, a higher level of storage reliability has been recently desired. It is difficult for the above technology to meet this desire.